Mass Spectrometry of Elements

Define Mass Spectrometry

Let's start by defining mass spectrometry.

Most mass spectrometers use a technique called electron impact ionization (EI). This technique uses a beam of electrons to remove an electron (or electrons) from a molecule, forming a radical cation. This radical cation is also called the parent ion or the molecular ion.

Curious about how a mass spectrometer works? Let's take a look!

Suppose that you have a sample of methane (CH₄). First, the gas sample will be inserted into the spectrometer. Then, electrons are beamed at the gas molecules, which might break into fragments and create radical cations. These charged particles travel to an ion detector after a deflection by a strong magnetic field that splits the ions based on their mass-to-charge ratio (m/z).

As the magnetic field increases, the ions of heavier isotopes move toward the ion detector. This ion detector is connected to a computer, showing the mass spectrum of the sample. The ion detector can detect their mass-to-charge ratio (which is their atomic mass), and the number of atoms that arrive at the detector for each m/z value.

Atomic Mass Spectroscopy

When the computer reads the collected data from the ion detector, it creates a mass spectrum that tells us the number of components in the sample, the relative molecular mass of each component, and their relative abundance.

Let's look at the mass spectrum of natural copper. This mass spectrum shows us the existence of two isotopes of copper. Specifically, it tells us the fractional abundances of both isotopes. So, by looking at the mass spectrum, we can tell that elemental copper is 69.17% ⁶³Cu and 30.83% ⁶⁵Cu.

Fig. 2: Mass Spectrum of Copper (Cu), StudySmarter Originals.

If you are still feeling confused, don't stress! We will look at more examples involving mass spectrum graphs in a bit.

Mass Spectrometry Equation

Mass spectrometry explains the relationship between the mass spectrum of an element and the masses of the isotopes of that element. Remember: elements occur in nature as mixtures of isotopes.

We can use the information from mass spectroscopy to calculate the relative atomic mass of an element, using the equation below:

$$A_{r}=\frac{\Sigma (isotope\ mass\cdot isotope\abundance)}{100}$$

Let's look at the mass spectrum of neon. Notice that it shows three peaks, or three isotopes of neon: ²⁰Ne (90.9% abundance), ²¹Ne (0.3% abundance), and ²²Ne (8.8% abundance).

Fig. 3: Mass Spectrum of Neon (Ne), StudySmarter Original.

We can use the equation above to find out the relative atomic mass of Neon.

$$A_{r}=\frac{\Sigma (isotope\ mass\cdot isotope\abundance)}{100}$$

$$A_{r}\ of\ neon\ (Ne)=\frac{(20\cdot 90.9)+(21\cdot 0.3)+(22\cdot 8.8)}{100}=20.2$$

Mass Spectroscopy: Graphs of Elements

Before looking at some examples of questions related to mass spectrometry that you might see in your exam, let's look at the graphs (mass spectrum) of some elements!

Fluorine and Iodine only have one peak on their mass spectrum. So, both ¹⁹F and ¹²⁷I have 100% abundance. On the other hand, Elemental bromine has two peaks, so it is 51% bromine-79 and 49% bromine-81. Chlorine also has two peak on its mass spectrum: 76% ³⁵Cl, and 24% ³⁷Cl

Fig. 5: Mass Spectra of Halogens, StudySmarter Original.

The mass spectrum of selenium is shown below. Can you guess how many isotopes of selenium (Se) exist? If you said 5, then you are right!

Fig. 6: Mass spectrum of Selenium, StudySmarter Original.

Mass Spectrometry Examples

Now that we know how to analyze a mass spectrum, let's look at some examples of exam questions.

Let's solve a different question.